1. Field of the Invention
The present invention relates to flat tension mask (FTM) color cathode ray tubes (CRT's) having the mask support structure affixed to the faceplate thereof. The present invention relates more specifically to means for attaining the accuracy necessary for predetermining the height of the "Q"-height mask support structures.
2. Discussion of the Related Art
Those familiar with the art of phosphor screen application to the faceplate of a common color cathode ray tube are aware of the advantages to be gained in production processes by making it possible to use shadow masks interchangeably with any screened faceplate. The assignee of the present invention manufactures a flat tension mask (FTM) color CRT. With the advent of the FTM, which utilizes a flat faceplate in conjunction with a thin foil shadow mask affixed in tension to mask support structures bonded to the flat faceplate, interchangeable mask technology is now practical. However, in order to meet the necessary registration demands for high resolution displays, the mask to screen distance, or "Q"-height, of the mask support structures must be maintained within very close tolerances. Due to dimensional inconsistencies of rail components as well as inconsistencies of the rail system affixation process, a procedure for grinding the mask support surface of the rail/panel assembly is considered a practical necessity. Indeed, in an interchangeable mask system utilizing offset printing to deposit the standard screen there are none of the mask to screen compensation factors inherent in photolithographic screen deposition. Therefore, "Q"-height control is critical to the functioning of such tubes.
In the past, it has been proposed to attain the necessary constancy of "Q"-height over the length of the mask support system by deforming the edges of the panel into intimacy with a true reference plane and then passing an abrasive grinding surface located in a parallel plane and at "Q"-height distance, over the rails to ensure the proper "Q"-height. See, e.g., U.S. Pat. No. 4,908,995, commonly owned herewith. However, the fixturing required for such a system may induce large forces and cause damage to the panel glass if the panel plane reference points are too hard, or introduce inconsistencies in "Q"-height if the reference points are subject to wear or deformation. Further, the "Q"-height measurements of such a system are based on the placement of the panel face periphery against reference stops. An assumption exists in this sort of grinding that the screen area of the panel is thereby placed in the same plane as the panel face periphery, resulting in adequate "Q"-height control. Greater accuracy of "Q"-height control can be attained by foregoing this assumption and instead measuring rail height from the periphery of the screen area resulting in a more effective "Q"-height measurement.
Therefore, it is desirable to provide a grinder which can establish a constant "Q"-height without requiring potentially destructive deformation of the panel.
Such a constant "Q"-height front panel, produced with nondestructive fixturing means, is particularly useful in a standardized screen printing system, such as elaborated in the parent 30 application U.S. Ser. No. 07/655,561, filed Feb. 13, 1991, now U.S. Pat. No. 5,158,491 and even more so in the aforementioned offset printing of screens. Said parent application is herein incorporated by reference to avoid lengthy exposition of background unnecessary to those ordinarily skilled in the art. It is therefor an object of the present invention to attain an accurate closely controlled "Q"-height for the mask support system affixed to the faceplate, as described above, with minimal injurious fixturing required during rail grinding.